This invention relates to masking a portion of the inner surface of a tubular, porous fuel cell support tube during vacuum deposition of an exterior electrode, to provide a taper on the end of the deposited exterior electrode near an end of the support tube.
Solid oxide electrolyte, high temperature, tubular fuel cells, and fuel cell generators employing such cells are well known, and taught by U.S. Pat. Nos. 4,490,444 and 4,395,468, respectively (both Isenberg). The cell contains a porous ceramic support tube of, for example, calcia stabilized zirconia, having an open and a closed end, covered in succession by: a thin film porous air electrode of, for example, doped LaMnO.sub.3, applied by aqueous vacuum slurry deposition and sintering; solid oxide electrolyte of, for example, yttria stabilized zirconia, covering most of the air electrode; and porous cermet fuel electrode covering the electrolyte.
Air electrode deposits can be made to cover the entire tube including the closed end, exclusive of about 7.6 cm near the open end; or the middle portion of the tube exclusive of about 2.5 cm near the closed end and about 7.6 cm near the open end, although these dimensions are not critical in any way. This deposition can be accomplished by appropriately coating a selected portion of both the inside and the outside of the tube with a masking material which will vaporize at over about 900.degree. C. Dipping the closed end of a tube into a solution of cellulose acetate is one possibility, to provide a masking film after drying. A vacuum would then be drawn on the inside of the support tube after submerging it in an aqueous slurry of finely divided air electrode powder.
The masking film is impermeable to the water and vacuum, therefore no air electrode powder is deposited in the masked areas. However, the deposit forms at the edge of the masking in an abrupt fashion, and may even grow over the edge of the masking, forming an angle of greater than 90.degree. with respect to the tube center surface. When this type of deposit is sintered, the porous support tubes, especially "thin-wall" porous support tubes (1.0 mm to 1.4 mm wall thickness) can break easily at the sharp joint between the tube and air electrode. Bending stress as low as 50.7 kg/cm.sup.2 (719 psi) up to 444.2 kg/cm.sup.2 (6,300 psi) have been measured on tubes with sintered air electrodes made by this process. Some tubes are broken simply by handling. The ragged edges can be machined to a taper after air electrode sintering, but such machining is time consuming, causes dust, and can cause tube fracture itself.
General immersion vacuum impregnation of unmasked porous tube walls is taught in Ewing U.S. Pat. No. 2,730,462, where a porous, closed ended tube is immersed in impregnant, air is evacuated from the tube interior and wall surfaces, and one end of the tube is opened to admit impregnant and fill the tube. More Larsh U.S. Pat. No. 3,028,266, where an inflatable, rubber like air bag is placed at the mouth of and along the complete length of slots in stator windings, to prevent resin contact at the mouth of the slots during wire varnish vacuum impregnation. Neither of these methods address the problems of support tube masking to provide exterior, smooth edge electrode coatings after vacuum impregnation.
There has been a long felt need for a simple, inexpensive method to properly mask application of electrode material on a support tube and eliminate abrupt and ragged edges of the electrode ends which lowers bending stress and creates handling problems.
A main object of this invention will be to provide such a method.